The management of sewage wastes necessitates the determination of the accumulation of separated sludge in sewage settling tanks. In accordance with one method, a photometric device is used. Such device comprises a light source and a light detector opposed to the source across a gap into which the liquid of the tank may freely flow. In one such apparatus a single device contained in a probe is lowered into the tank manually or by a winding mechanism, and when a predetermined attenuation of the detector output is measured, it is considered that the probe has entered the sludge bed. In another apparatus a fixed mast is employed having three photometric devices axially spaced thereon to given an indication of whether the sludge bed is at a low, medium or high level; signal from the apparatus may be used to automatically control the sludge level in the tank.
Certain disadvantages are attendant upon the use of the foregoing apparatus. Thus both necessitate the use of skilled operators, and rely significantly upon the interpretation of the measurements outputted from the devices. Such measurements may well be spurious due to local disturbances in the settled solids, due to solid materials floating in the liquid and air bubbles, for example. The mechanical apparatus is subject to malfunction, particularly when used under freezing conditions. The fixed probe gives little information regarding the rate of sludge build up, and in addition an accumulation of scum on the windows behind which the component parts of the photometric devices locate rapidly attenuates the detector outputs, necessitating relatively frequent withdrawal of the mast to clean the windows. The fixed probe must also be removed and the photometric devices relocated thereon when it is wished to change the depth settings of the sludge to be measured and controlled in the tank.
In accordance with another method, a sonar device is used. However, the composition of a typical sludge bed often comprise not more than about 1% solids, hence there is little material from which to reflect sound waves. Moreover, when the sludge is separated by a froth flotation technique, the sonar method is rendered unusable since the air bubbles absorb the sonic signal. It may also be noted here that the traditional photometric apparatus is not very effective for use in connection with froth floatation methods of separating solids.
It is a primary object of this invention to provide improved apparatus for determining the accumulation of separated sludge in sewage tanks or the like.
It is a further object of the invention to provide automated apparatus of the foregoing type.
It is another object of the invention to provide apparatus which does not involve mechanically moving parts and that is consequently more reliable in operation and less susceptible to freeze up problems.
It is another object of the invention to provide apparatus which provides an output that is less susceptible to local variations that may be encountered in the fluid with which the apparatus is employed.
It is another object of the invention to provide apparatus wherein means is incorporated for compensating for scum build up on the probe.
It is another object of the invention to provide apparatus wherein the depth settings to be measured and controlled can be easily varied.
It is another object of the invention to provide apparatus for use in connection with froth floatation sludge separation techniques.